This method relates to an improved electrode apparatus and method for electrically heating a subterranean formation. In another aspect, this invention relates to an improved electrode apparatus and method for conducting electric current into a subterranean formation with improved and increased contact between the conducting elements and the subterranean formation. In still another aspect, this invention relates to an improved method and apparatus for uniformly heating a subterranean formation by passing an electric current through said formation between spaced-apart electrode means.
For many years, it has been known that large deposits of very viscous material, such as tar, heavy crude oil, and the like, are present in subterranean formations. Because of the high viscosity of some of these materials, various methods for heating them in situ to lower their viscosity, have been suggested. By lowering the viscosity of such materials as tar, which can be found in large deposits of subterranean tar sands, the materials can be produced through production wells by means of injecting certain driving fluids as steam, hot water, hot gases and the like. Normally, in order to carry out such a production technique, it is necessary to first heat at least a portion of the subterranean formation to lower the viscosity of the viscous material to a point where the driving fluid can initiate flow of the material from the subterranean formation.
Recently, techniques have been utilized that incorporate the use of electric currents to pass through the subterranean formation. As the electric currents pass through the subterranean formations, the inherent resistance of the formations will cause the formations to heat up and thereby lower the viscosity of the viscous materials contained therein.
Since the discovery of the method of passing electric currents through the subterranean formations to lower the viscosity of the materials contained therein, a considerable amount of activity has been devoted to developing techniques using this basic process. For example, process and apparatus using this basic discovery have been disclosed in U.S. Pat. Nos. 3,642,066, issued Feb. 15, 1972; 3,874,450, issued Apr. 1, 1975; 3,848,671, issued Nov. 19, 1974; 3,948,319, issued Apr. 6, 1976; and 3,958,636, issued May 25, 1976, all of which Patents are hereby incorporated by reference.
While the foregoing patents represent only a few of the techniques that utilize electrodes for passing current therebetween to heat subterranean formations in situ, these patents and various others all recognize certain problems and dificulties in evenly conducting electricity through such subterranean formations. For example, most of the references acknowledge the fact that large amounts of current must be passed through the subterranean formations in order to achieve the desired heating. With the passage of large amounts of current through such formations, it is also recognized that the portions of the subterranean formations immediately adjacent the electrodes experience the greatest current densities. As the current density or the amount of current flowing through a given area is increased, a problem of overheating in the general area is experienced. The overheating problem becomes so severe in some cases that it will dry out or vaporize electrolyte materials in the general vicinity of the electrodes, thus causing an interruption or decrease in the amount of current that flows through the subterranean foundations. Additionally, the excessive heat will often be so great as to melt or otherwise damage the electrode members. It is further recognized in the prior art that it is often difficult to obtain a good, conductive contact between the electrode members placed in electrode wells and the surrounding subterranean formation that is to be heated.
In view of the foregoing problems and deficiencies of prior art methods and apparatus for passing current through subterranean formations, it is, of course, highly desirable to develop improved methods and apparatus for such use.